1. Field of the Invention
This invention relates to a method for repairing post and pre-tensioned concrete structures such as parking ramps whose beams are damaged. More particularly, the method involves the creation of a new post-tensioned beam in a trench where the old tendon and encasing concrete were present.
2. Description of the Related Art
Post-tensioned concrete structures depend on the spaced beams of concrete in which a post-tensioned tendon is positioned. Pre-tensioned structures apply tension to the tendon prior to the concrete pour. With time, salt and other detrimental chemicals tend to break down the concrete and contacts the tendons. Damage to the tendons may severely weaken the integrity of the post-tensioned structure requiring repair. In some cases damage is so severe that the entire deck must be replaced which is extremely expensive and effectively puts the structure out of use until repairs are completed.
An alternative method of repairing such post-tensioned structures is described in Reigstad et al, U.S. Pat. No. 4,574,545, the disclosure of which is incorporated herein by reference. In Reigstad, concrete is repaired either by pulling the individual strands of the steel tendon out and replacing the tendon after reaming the bore, or by releasing tension in the tendon and exposing the tendon from above at its ends and beneath through its central span. After the tendon is removed, new concrete is applied to cover the new tendon. Plywood must be applied to the underside of the slab and be supported overhead by shoring from the slab below as a form for the new concrete placed on the ceiling of the slab.
The Reigstad process involves the use of jackhammers and overhead jackhammer stands from below which remove concrete from the ceiling. The method necessitates the closing of both the floor being worked on and the lower floor where jackhammers must be used and forming must be supported by shoring. The Reigstad process leaves much more than half of the original concrete untouched around the tendon being replaced, for fear that excessive removal of concrete e will cause slab deformation. Thus, a large amount of concrete above the tendon, which may have chloride ions present, is left in place. The new tendon in the Reigstad process is coated to prevent corrosion by this and other sources of chloride ions.
The Reigstad process is particularly inappropriate when the slab being repaired functions as a ceiling for office or retail store space, since such spaces would need to be closed down during the repair process. Unfortunately, complete slab replacement or complete removal of the post-tensioned beams also require the shutdown of any underlying businesses.
Reigstad describes repair methods in which the entire beam is removed by cutting completely through the slab. This method was characterized as being unworkable. The Reigstad process had been known and practiced in the United States since at least as early as 1970.